Seymour was born in Hartford, Connecticut, and her parents died while she was a teenager. Prior to her mother's death, she was adopted by the family of the American Civil War veteran and social activist Lloyd G. Seymour.

In June 1888 she visited the City Hall to review her birth certificates and declared her official name as Mary Emma Townsend Seymour. In 1891, she married Frederick Seymour, a member of the Seymour family.[2]

Many African Americans at the time had migrated north from the southern United States. In Hartford, school officials considered segregating the schools. Seymour and her husband, along with 20 other local residents, began organizing for civil rights. On October 9, 1917, the Hartford chapter of the National Association for the Advancement of Colored People (NAACP) was founded, with Seymour serving as its spokesperson.

Around 1920, Seymour joined the American Red Cross and worked with African American women tobacco packers. She and fellow activist Josephine Bennett interviewed female laborers, and co-wrote an exposé on the subject for the NAACP's The Crisis in 1920.[2]

In 1920, Seymour became the first African American woman to run for the Connecticut General Assembly. She did not win, but was the first African American woman to run for this position.

She died in Hartford in 1957 and is buried in Hartford's Old North Cemetery. Her grave is a site on the Connecticut Freedom Trail.

In 1997 Mary Seymour Place was opened in Hartford, to provide supportive housing for homeless women and children.